Phase-locked loops (PLLs) are used in data communications and telecommunications applications to lock onto the frequency of an incoming signal. A typical PLL includes a voltage controlled oscillator (VCO) that is tuned to match the frequency of the incoming signal. One technique that is used to reduce the magnitude of the required tuning gain of a VCO and to reduce the corresponding noise sensitivity of the VCO while still providing a wide frequency range involves splitting the entire frequency tuning range of the VCO into multiple overlapping frequency bands. VCOs that are configured with multiple overlapping frequency bands are generally referred to as “multi-band VCOs.” FIG. 1 depicts an example graph of VCO frequency vs. the VCO tuning signal voltage for multiple different frequency bands 90 of a multi-band VCO.
Controlling a multi-band VCO to lock onto a setpoint frequency involves “centering” the VCO by selecting a frequency band that includes the setpoint frequency. One technique for centering a multi-band VCO is described in the U.S. patent application entitled “Centering a multi-band voltage controlled oscillator,” Ser. No. 10/775,960, filed Feb. 10, 2004, which is assigned to the assignee of the current application and incorporated by reference herein. The technique involves comparing a VCO tuning signal to a pre-established tuning signal window to determine whether to change the frequency band of the multi-band VCO. Using this technique, if the pre-established tuning signal window is set too narrow, there may not be a frequency band that intersects the setpoint frequency at a point that is within the tuning signal window. If there is no operating point that meets the setpoint frequency and tuning signal window requirements, then the VCO will begin to “churn” around the setpoint frequency. That is, the trajectory of the VCO frequency will continuously move back and forth across the tuning signal window and around the setpoint frequency without finding a solution, where a solution is defined as an operating point along a frequency band that satisfies both the setpoint frequency and tuning signal window requirements.
Churning can be prevented by simply setting the tuning signal window of a multi-band VCO so wide that there is always a solution. The problem with setting the tuning signal window so wide is that there will likely be more than one frequency band that intersects the setpoint frequency within the tuning signal window. When there are multiple intersecting frequency bands within the tuning signal window, there is no guarantee that the VCO will converge to the best solution.
The ideal tuning signal window for a multi-band VCO is a function of the setpoint frequency, the spacing of the frequency bands, and operating conditions of the PLL. One way to determine the ideal tuning signal window for a multi-band VCO is by carrying out simulations taking into account all process corners. A drawback to simulations is that they are a time consuming and expensive proposition. Further, the results of any simulations must be generalized to different sets of operating conditions.